What is the minimum air space behind siding in millimeters?

• A. 3 mm
• B. 6 mm
• C. 9.5 mm
• D. 12 mm

Answer: (C)

Ventilation and drainage behind siding are needed to dry and vent any moisture that gets behind the outer layer. The minimum air space that accomplishes this effectively is 9.5 mm (3/8 inch). Gaps smaller than this, such as 3 mm or 6 mm, restrict airflow and drainage, increasing the risk of moisture buildup, rot, and mold. A gap of 12 mm is larger than the minimum required and isn’t the prescribed minimum for this scenario, though it could be used in some rain-screen designs.